The present invention relates to a quick-return motion mechanism for use in automatic magnetic drilling machines.
One known quick-return motion mechanism for use in automatic magnetic drilling machines is constructed as depicted in FIG. 1, wherein power generated from feed motor 1 is transferred through a pinion 1a, a first gear (toothed-wheel) 2, a worm 3, a worm wheel 4, a high-ratio reduction gear 5 and clutches 6, 7 to a handle shaft 8 so as to move an electric drill up and down. The worm wheel 4 is fixed to the input side of the high-ratio reduction gear 5. A one-way clutch 9 is disposed inside the worm wheel 4 such that the handle shaft 8 races on feeding and is locked on returning. The high-ratio reduction gear 5 is arranged s as to take the same rotational direction at the input and output sides, and the clutch 6 is fixed to the output side of the high-ratio reduction gear 5. As depicted in FIG. 2, the engaging surfaces 6a and 7a of the clutches 6 and 7 are engaged with each other with a given angle .theta. so that a spring 10 is compressed in response to application of a certain transmission torque and hence the clutch 7 retreats in the rotational direction so as to slip. In FIG. 1, numeral 11 represents a stopper for the clutch 7, 5a designates elements, 12 depicts a gear cover, and 13 denotes a bolt.
With reference to FIG. 3 when the feed motor 1 rotates in the left direction (clockwise), the first gear 2 rotates in the right direction (counterclockwise), so that power is transferred to the worm wheel 4, the high-ratio reduction gear 5 and the clutches 6, 7 whereby the handle shaft 8 rotates in the right direction. The worm wheel 4 relatively rotates in the right direction with respect to the handle shaft 8. In this case, the clutches 6 and 7 engage each other. The reduction ratio up to the handle shaft 8 is i1 .times. i2 .times. i3 and the rotational speed is NO/i1, i2, i3, where i1 is the reduction ratio of the pinion 1a and the first gear 2, i2 is the reduction ratio of the worm 3, i3 is the reduction ratio of the high-ratio reduction gear, and NO is the rotational speed of the feed motor 1. On returning, in response to the right rotation of the feed motor 1, the first gear 2 rotates in the left direction and the power is transferred to the worm wheel 4 and the high-ratio reduction gear 5 whereby the clutch 6 rotates in the left direction with a rotational speed of NO/i1, i2, i3. On the other hand, the one-way clutch 9 causes the worm wheel 4 and the handle shaft 8 to be locked, and the handle shaft 8 and the clutch 7 rotate at a rotational speed of NO/i1, i2 which is equal to that of the worm wheel 4. The clutch 6 is locked with the clutch 7 which in turn, slips when exceeding the slip torque, and the handle shaft 8 rotates at a rotational speed of NO/i1, i2.
Although as described above the rotational speed on returning is i3 times of the rotational speed on the feeding so as to increase the rotational speed of the feed motor 1, there is a problem which arises with such a mechanism, i.e., when the thrust of the electric drill is great, the clutch starts to slip so as to make drilling difficult. Further, if increasing the slip torque in correspondance with the drilling thrust, the feed motor 1 is locked on return so as to make difficult the quick-return operation.